Miscible-displacement process



3,196,944 MISCIBLE-DISPLACEMENT PROCESS George G. Bernard and Orrin C.Holbrook, Crystal Lake, [1]., assignors to The Pure Oil Company,Chicago, 11].,

a corporation of Ohio No Drawing. Filed Dec. 11, 1961, Ser. No. 158,5757 Claims. (Cl. 166-9) This invention relates to the recovery ofpetroleum from subterranean reservoirs. It is more specificallyconcerned with improving the efficiency o f gas-drive, secondaryr e;covery operations.

Secondary recovery of petroleum by the injection of a gas through aninput well and withdrawal of reservoir fluids from a producing Well haslong been known and practiced. It was later discovered, in laboratorycore experiments, that the recovery of petroleum by gas-drive could begreatly increased by first injecting a liquid hydrocarbon of lowviscosity through the injection well, and

following this slug of liquid hydrocarbon with a gas which is misciblewith or highly soluble in the liquid hydrocarbon. Thus, it was found,for example, that when a petroleum-containing core was treated by theinjection of a small amount of low-molecular-weight hydrocarbon, such asliquid propane, and then driven by a gas which is miscible with thepropane, such as natural gas or methane, a highly efiicient process wasobtained.

Oil recoveries achieved in applying such miscibledisplacement gas-driveprocesses to actual reservoirs were disappointing, in that the total oilrecovery fell far short of that anticipated from predictions based oncore experiments. It was found that while the process in fact succeedsin recovering practically all of the petroleum in the reservoir which iscontacted by the injected liquid hydrocarbon, large quantities ofpetroleum are still by-passed and left in the reservoir when the processis carried out. This unfortunate result is due to the unfavorablemobility relationships of the reservoir fluids and injected fluids,which results in unsatisfactory areal sweep efficiencies and acorresponding decrease in the over-all efficiency of the process.

It is an object of this invention to provide an improvedmiscible-displacement, gas-drive, secondary-recovery proc cess by whichgreater quantities of oil can be produced. Another object of thisinvention is to provide a miscibledisplacement, gas-drive process inwhich greatly enhanced areal sweep efficiencies are obtained.

It has ben found that the areal sweep efliciency ofmiscible-displacement, gas-drive, secondary-recovery processes can begreatly improved, and the over-all efficiency of the processes therebyenhanced, by incorporating in the low-viscosity liquid hydrocarboninjected a small amount of a high-foaming surfactant. The surfactantmust be oilsoluble, and of the high-foaming type as hereinafter defined.It has been found that the injected driving gas coacts with thesurfactant-containing liquid hydrocarbon to produce a stable foam whichappears as a bank at the trailing edge of the bank of injected liquidhydrocarbon.

In carrying out the process of this invention, about 0.03 to 0.20reservoir pore volume of a low-viscosity liquid hydrocarbon is firstinjected through the input well and into the reservoir. The liquidhydrocarbon is preferably a liquefied, normally gaseous,low-molecular-weight material, such as L.P.G., propane, or butane.However, other low-molecular-weight, normally liquid hydrocarbonfractions may be used. In general, hydrocarbon fractions consistingpredominantly of C and lower hydrocarbons are satisfactory. The gasinjected to provide a gas drive must be inert, and miscible with, orhighly soluble in, the injected hydrocarbon. Gases such as methane,ethane, and natural gas are preferred. The oil-soluble, high-foamingsurfactant should be incorporated in the United States Patent 0 SEARm-iWWW 3,196,944 Patented July 27, 1965 ice first-injected liquidhydrocarbon in the amount of about 0.1 to 5% by weight. The exact amountwill vary de pending upon the nature of the surfactant injected, butshould be suflicient to produce a substantial quantity of stable foamupon contact of the surfactant-containing liquid hydrocarbon withnatural gas. The exact amount of surfactant to be used can be determinedby experiment, as hereinafter described.

The term high-foaming surfactant, as used in this specification and theappended claims, denotes a surfactant which, when incorporated in aliquid hydrocarbon in an amount not in excess of 5% by weight, meets thefollow ing test. The surfactant is dissolved in a hydrocarbon, such ashexane, and 500 ml. of the solution is placed in a graduated cylinder toform a column having a height of 50 centimeters. Natural gas at lowpressure is passed into the bottom of the column through a fritted-glassdisc at substantially atmospheric pressure, so that the gas bubblesthrough the column of liquid and passes out of the top of the cylinder.The gas rate is maintained at about 500 ml. of gas per minute, persquare inch of column cross-sectional area. The flow of gas is continuedfor a period of 15 minutes, after which time the flow of gas isdiscontinued. A column of foam will then be found to exist at the top ofthe column of liquid hydrocarbon. A high-foaming surfactant, as definedin this specification, must be capable of producing a column of foam notless than centimeters in height under the conditions aforedescribed.

With some high-foaming surfactants, the afore-described 3;;

hydrocarbon. Thus, where it is found that 2% by weight of a givensurfactant is capable of meeting the requirements of the afore-definedexperiment, it is preferred, for reasons of economy, that this amount beused. The use of excess quantities of surfactant should be avoided forreasons of economy, and to prevent the production of an excessivelythick and viscous foam bank, which results in the necessity foremploying excessive gas-injection pressures. While the use of variouscommercial high-foaming surfactants is contemplated, an example of asatisfactory foaming agent is a 1-to-1 mixture of di-coco dimethylammonium chloride and decyl trimethyl ammonium chloride. This mixturecan be used in the amount of about 2% by weight, with excellent results.High-foaming surfactants can be selected from published tablesdescribing the properties of commercially available surfactants.

As a speci figgxample of the method of this invention, apetroleum-containing reservoir is subjected to secondary recovery byinjecting through an input well a s lpgpf liquid propane in which isincorporated 2% by weight of a 1-to-1 mixture of di-coco dimethylammonium chloride and decyl trimethyl ammonium chloride. Natural ga s isthen injected at sufiicient pressure to maintain the propane as aliquid, to drive the propane bank toward four producing wells whichsurround the injection well. Petroleum is produced from the producingwells until the gas-to-oil ratio reaches an unattractively high value.At this point, the injection of gas is terminated and productioncontinued under conditions of pressure depletion.

While allifi the injected liquid hydrocarbon may contain a high-foamingsurfactant, and such is preferred where small amounts of liquidhydrocarbon are injected, when larger banks of liquid hydrocarbon areused, it is preferred that the surfactant be incorporated (mlyim thelast-injected portion of the liquid hydrocarbon. Thus, where thequantity of liquid hydrocarbon l entire quantity of liquid hydrocarbon,or at least in most j' of it. Where a largerbank of propane is employed,such as 0.15 reservoir pore volume, it is preferred that thesurfactanb-he incorporated only in the last-injected por-, tion of theliquid hydrocarbon. The amount of liquid hydrocarbon in which thesurfactant is placed should be not less than about 0.03 reservoir porevolume, and ordinarily will be about 0.05 reservoir pore volume. Wherethe reservoir is known to contain substantial quantities of free gasdispersed in a substantially homogeneous distribution throughout thereservoir, it is especially preferred that the first-injected liquidhydrocarbon contain no surfactant in order to avoid the production of abank of foam at the leading edge of the liquid hydrocarbon bank, therebyencouraging the liquid hydrocarbon to bypass a portion of the reservoir.On the other hand, where the reservoir is known to contain free gaswhich is localized in a gas cap above the petroleum-containing portionsof the reservoir, it is advantageous to distribute the surfactantthroughout the entire liquid hydrocarbon bank, so that the production ofa foam bank at the interface with the gas cap will occur, thus retardingthe entry of the liquid hydrocarbon bank into the gas cap, and avoidingconsequent bypassing of the petroleum-containing portions of theformation.

' The objects of this invention cannot be achieved by incorporating thesurfactant in the injected gas, rather than in the liquid hydrocarbon,for the reason that any foam produced will lag behind the liquidhydrocarbongas interface, and the presence of the foam at this interfaceis essential.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. The method for recovering petroleum from underground reservoirspenetrated by an injection well and a 2. The method in accordance withclaim 1 in which the amount of liquid hydrocarbon injected is in excessof 0.05 reservoir pore volume, and the portion of said liquidhydrocarbon in which the high-foaming surfactant is incorporated is notin excess of 0.05 reservoir pore volume.

3. The method in accordance with claim 1 in which the high-foamingsurfactant is in hydrocarbon solution in the amount of about 0.1 to 5.0%by weight.

4. The method in accordance with claim 1 in which the liquid hydrocarbonis liquefied propane, and the hydrocarbon gas is injected undersufficient pressure to maintain said propane as a liquid in thereservoir.

- 5. The method in accordance with claim 4 in which the concentration ofhigh-foaming surfactant in said propane is about 0.1 to 5.0% by weight.

6. The method in accordance with claim 5 in which the quantity ofpropane injected is in excess of 0.05 reservoir pore volume, and thelast-injected portion of said bank in which high-foaming surfactant isincorporated amounts to about 0.05 reservoir per volume.

7. The method in accordance with claim 6 in which said high-foamingsurfactant is a 1-to-1 mixture of dicoco dimethyl ammonium chloride anddecyl trimethyl producing well comprising injecting through saidinjection well and into said formation 0.03 to 0.20 reservoir porevolume of a low-molecular-weight liquid hydrocarbon, at least thelast-injected 0.03 reservoir pQIe. volume.

of which contains in solution ghigh-fogming, oil-soluble V surfactant inan amount suflicient to produce a foam on contact in the reservoir witha hydrocarbon gas, win; jecting hydrocarbon gas, substantially free of asurfactant, to form a foam bank at least between said injected liquidhydrocarbon and said hydrocarbon gas and to drive said injected liquidhydrocarbon toward said producing well, and recovering petroleumtherefrom.

ammonium chloride, and the mixture is incorporated in the propane in theamount of about 2% by weight.

References Cited by the Examiner UNITED STATES PATENTS Armour andCompany publication, received in Patent Officer, 1954.

Dunning:

Using Foaming Agents To Remove Liquids From Gas Wells, Monograph 11,Bureau of Mines. Published 1961, 166-45 (38 pages).

5 CHARLES E. OCON-NELL, Primary Examiner.

NOR AN YU OFF, Examin

1. THE METHOD FOR RECOVERING PETROLEUM FROM UNDERGROUND RESERVOIRS PENETRATED BY AN INJECTION WELL AND A PRODUCING WELL COMPRISING INJECTING THROUGH SAID INJECTION WELL AND INTO SAID FORMATION 0.03 TO 0.20 RESERVOIR PORE VOLUME OF A LOW-MOLECULAR-WEIGHT LIQUID HYDROCARBON, AT LEAST THE LAST-INJECTED 0.03 RESERVOIR PORE VOLUME OF WHICH CONTAINS IN SOLUTION A HIGH-FOAMING, OIL-SOLUBLE SURFACTANT IN AN AMOUNT SUFFICIENT TO PRODUCE A FOAM ON CONTACT IN THE RESERVOIR WITH A HYDROCARHBON GAS, THEN INJECTING HYDROCARBON GAS, SUBSTANTIALLY FREE OF A SURFACTANT, TO FORM A FOAM BANK AT LEAST BETWEN SAID INJECTED LIQUID HYDROCARBON AND SAID HYDROCARBON GAS AND TO DRIVE SAID INJECTED LIQUID HYDROCARBON TOWARD SAID PRODUCING WELL, AND RECOVERING PETROLEUM THEREFROM. 